Cryogenic liquefaction is commonly used to convert natural gas into a more convenient form for transportation and/or storage. Because liquefying natural gas greatly reduces its specific volume, large quantities of natural gas can be economically transported and/or stored in liquefied form.
Transporting natural gas in its liquefied form can effectively link a natural gas source with a distant market when the source and market are not connected by a pipeline. This situation commonly arises when the source of natural gas and the market for the natural gas are separated by large bodies of water. In such cases, liquefied natural gas (LNG) can be transported from the source to the market using specially designed ocean-going LNG tankers.
Storing natural gas in its liquefied form can help balance periodic fluctuations in natural gas supply and demand. In particular, LNG can be “stockpiled” for use when natural gas demand is low and/or supply is high. As a result, future demand peaks can be met with LNG from storage, which can be vaporized as demand requires.
Several methods exist for liquefying natural gas. Some methods produce a pressurized LNG (PLNG) product that is useful, but requires expensive pressure-containing vessels for storage and transportation. Other methods produce an LNG product having a pressure at or near atmospheric pressure. In general, these non-pressurized LNG production methods involve cooling a high pressure natural gas stream through indirect heat exchange with one or more refrigerants and then expanding the cooled natural gas stream to near atmospheric pressure. In addition, some LNG facilities employ one or more systems to remove contaminants (e.g., water, mercury and mercury components, acid gases, and nitrogen, as well as a portion of ethane and heavier components) from the natural gas stream at different points during the liquefaction process.
In certain LNG facilities, an inlet gas stream is combined with one or more lower pressure refrigerants into a single combined stream that is then further cooled and processed. The combined stream is then fed to a nitrogen rejection unit (NRU), used as fuel gas, and/or processed further. In the case of methane as the refrigerant, the higher concentration of nitrogen in the methane stream is diluted upon combination with the inlet gas stream. Dilution requires a larger volume of the combined stream to be sent through the NRU for processing to an acceptable amount of nitrogen. The combined stream feed volume required to be sent to the NRU impacts the equipment size and cost of the NRU.